Add comment to map_vis_z_ parameter

nav2_costmap_2d/include/nav2_costmap_2d/costmap_2d_ros.hpp

@@ -404,7 +404,7 @@ class Costmap2DROS : public nav2_util::LifecycleNode
   bool track_unknown_space_{false};
   double transform_tolerance_{0};           ///< The timeout before transform errors
   double initial_transform_timeout_{0};   ///< The timeout before activation of the node errors
-  double map_vis_z_{0};
+  double map_vis_z_{0};                 ///< The height of map, allows to avoid flickering at -0.008
 
   bool is_lifecycle_follower_{true};   ///< whether is a child-LifecycleNode or an independent node